Roller assemblies, including tires disposed at longitudinally spaced apart locations on opposed shafts and arranged to contact one another at a nip are commonly used to transport paper or other sheet materials in printers and copiers. Normally, such tires are either hard and relatively non-compliant in which case they must be very precisely aligned and spaced to transport substrates effectively, or, the tires are soft and compliant so that they run in a compressed state thereby reducing the requirements for accurate alignment and spacing.
To accommodate substrates of varying width, it is common to provide a pair of shafts each of having a plurality of tires thereon. The longitudinal spacing between the tires is set so that the narrowest substrate is transported by at least two tires, wherein additional tires contact progressively wider substrate.
Typically, the shafts are supported only at their ends. If non-compliant tires are used, not only must the shafts be maintained in a precisely parallel orientation, but the shafts must be sufficiently rigid to preclude the shaft from bowing, even as the substrate passes through the nip. Providing such shafts that are sufficiently straight and rigid, and aligning the shafts is difficult and increases cost while decreasing the reliability of transport mechanisms using non-compliant tires.
In an effort to overcome these problems, compliant tires that significantly deform at the nip have been employed. When confronting compliant tires deform, the radius of the tire changes and this reduces the speed at which the substrate is transported through the nip. When multiple compliant tires are mounted on a single shaft, and the spacing between the shaft is not uniform across the length of the shaft, or the amount of compression of the compliant tires differs from one tire set to the next, speed differentials between the tire sets are created that cause the substrate, such as the paper to skew.
Compliant tires have another disadvantage. Because the compliant tires contact the paper at a contact patch that has different radii relative to the shaft across the length of the contact patch, the speed of the tire surface relative to the shaft changes while the tire contacts the substrate and this creates a scrubbing action between the tire and the substrate that scuffs the paper and wears the surface of the tire. Scuffing of the substrate is particularly troublesome when printed or copied images are present on the substrate or with substrates that will be printed after transport. Printing is adversely effected by damage to the surface caused by scuffing.
Thus, both known tire constructions, compliant and non-compliant, create problems. Non-compliant tires must be very precisely manufactured and aligned. Tolerances in shaft spacing of 0.002″ are generally considered to be required. Non-compliant tires require hand installation and mounting, which further increases costs. In addition, the criticality of alignment requires frequent maintenance. While compliant tires place less strict requirements on alignment, the compliant tires introduce the problems of skewing and scuffing.
Therefore, a need exists for a sheet material transport system that overcomes these problems, reduces wear on the tires and scuffing of the paper, and is easy to manufacture and maintain in alignment. There is a further need for a roller that can be readily manufactured to include a plurality of tires. A need also exists for a roller that provides the advantages of non-compliant tires and compliant tires without the associated disadvantages.